The present invention relates to photoluminescent polarizers and in particular to photoluminescent polarizers which are characterized in a low degree of polarization in their absorption and a high degree of polarization in their emission. The invention also relates to methods to produce the latter. Also, the invention relates to the application of these photoluminescent polarizers in display devices.
Linear sheet polarizers, which convert unpolarized into linearly polarized light are well known in the art and are of major importance in a large variety of applications (L. K. M. Chan in xe2x80x9cThe Encyclopedia of Advanced Materialsxe2x80x9d, Vol. 2, D. Bloor, T. J. Brook, M. C. Flemings, S. Mahajan, eds., pp. 1294-1304 (1994), Elsevier Science Ltd., Oxford; D. S. Kliger et al. xe2x80x9cPolarized Light in Optics and Spectroscopyxe2x80x9d (1990), Academic Press, San Diego; T. J. Nelson et al. xe2x80x9cElectronic Information Display Technologiesxe2x80x9d (1997), World Scientific Publishing, Singapore). However, the presently employed polarizers suffer from severe limitations some which are summarized below.
The vast majority of linear sheet polarizers presently used, are dichroic polarizers which are based on an invention by Land et al. (E. H. Land, J. Opt. Soc. Am., Vol. 4, pp. 957 (1951)). As well established in the prior art, dichroic polarizers are produced from oriented, synthetic polymers which contain oriented dichroic species. Dichroic polarizers operate by the absorption of one polarization direction of incident light, thus, a dichroic polarizer which generates perfectly, linearly polarized light absorbs 50% or more of unpolarized, incident light (D. S. Kliger et al. xe2x80x9cPolarized Light in Optics and Spectroscopyxe2x80x9d (1990), Academic Press, San Diego). Consequently, dichroic polarizers convert at least 50% of the incident optical energy into heat which severely limits the efficiency of these polarizers and causes problems due to the excessive heating in combination with high-intensity light sources.
As an alternative to dichroic polarizers, polarizers have been proposed that are based on selective reflection or scattering of one polarization and allow recycling of the reflected or scattered light (European Patent EP 0 606 940 A2; World Patent WO 9735219 A1; U.S. Pat. Nos. 5,325,218; 5,422,756; 5,528,720; 5,559,634; M. Schadt et al., Jap. J. Appl. Phys., Vol. 29, pp. 1974-1984 (1990); D. J. Broer et al., Nature, Vol. 378, pp. 467-469 (1995); D. Coates et al., SID 96 Applications Digest, pp. 67-70 (1996)) or scattering (Y. Dirix, xe2x80x9cPolarizers based on anisotropic absorbance or scattering of lightxe2x80x9d, Ph. D. thesis, Technische Universiteit Eindhoven, Eindhoven. The Netherlands (1997)). However, these polarizers also suffer a number of severe drawbacks. All above referred reflecting or scattering polarizers, due to their working principle, require additional light-recycling systems and other additional elements which render them rather uneconomical. Some of these polarizers initially produce circularly rather than linearly polarized light (D. J. Broer et al., Nature, Vol. 378, pp. 467-469 (1995)) and require expensive quarter-wave converters to produce linearly polarized light, or are manufactured by processes with intrinsic limitations for the production of large area, flexible polarizing films.
As is well known in the art, the production of linearly polarized, chromatic (colored) light, which is essential for many technical applications, including liquid-crystal displays, presents another obstacle. Polarized, colored light is usually obtained by the use of multiple elements: a polarizer and one or multiple color filters (L. K M. Chan in xe2x80x9cThe Encyclopaedia of Advanced Materialsxe2x80x9d, Vol. 2, D. Bloor, T. J. Brook, M. C. Flemings, S. Mahajan, eds., pp. 1294-1304 (1994), Elsevier Science Ltd., Oxford). The vast majority of color filters presently used are absorbing color filters which convert a major fraction, i.e. usually 80%, of incident light into thermal energy (T. J. Nelson et al. xe2x80x9cElectronic Information Display Technologiesxe2x80x9d p. 244 (1997), World Scientific Publishing, Singapore) and, thus, also create severe limitations with respect to energy efficiency, brightness and accumulation of thermal energy. As an alternative to absorbing color filters, the use of photoluminescent (PL), for example fluorescent or phosphorescent matter as xe2x80x9cactivexe2x80x9d color filters has also been described (German patent No. DE 2640909 C2; French application FR 2 600 451-A1: U.S. Pat. Nos. 3,844,637; 4,113,360; 4,336,980; 4,394,068; 4,470,666; 4,678,285; 5,018,837; 5,608.554; G. Baur et al., Appl. Phys. Lett., Vol. 31, pp. 4-6 (1977); M. Bechtler et al., Electronics, December 8, pp. 113-116 (1977); W. Greubel et. al., Elektronik, pp. 55-56 (1977); H. J. Coles, Liq. Cryst., Vol. 14, pp. 1039-1045 (1993); W. A. Crossland et al., Proc. SID Symp. Digest of Technical Papers, Vol. 27, pp. 837-840 (1997)). However, the proposed structures suffer from a number of drawbacks that are related to the limited stability and efficiency of the fluorescent dyes the difficulty to produce structured materials, depolarization effects, or the required thickness and (large) area of the luminescent layer.
Recently, some PL materials have been demonstrated to combine the functions of a linear polarizer and a color filter and to yield linearly polarized, chromatic light in one single element (Ch. Weder et al., Adv. Mat., Vol. 9, pp. 1035-1039 (1997)). When processed into appropriate forms, these PL materials can be used as PL polarizers which lead to a substantial increase in device brightness and efficiency when used instead of a dichroic polarizer and an absorbing color filter, for example in liquid-crystal display devices. In addition, PL polarizers offer a significant simplification in device design, because they combine the functions of two elements. The prior art PL polarizers comprise uniaxially oriented, formanisotropic, PL substances, which after photoexcitation emit linearly polarized light. This effect is well known in the art; it was demonstrated in inorganic crystals more than a century ago (E. Lommel, Ann. d. Physik und Chemie. Vol. 8. pp. 634-640 (1879)) and in oriented blends of ductile polymers and low-molecular weight PL materials as early as the 1930""s (A. Jablonski, Acta Phys. Polon., Vol. A 14, pp. 421-434 (1934)). Since, the effect has been shown in a variety of systems (J. Michl et al. xe2x80x9cSpectroscopy with polarized lightxe2x80x9d (1986), VCH Publishers, New York) including, for example, oriented blends of ductile polymers and oligomeric PL materials (M. Hennecke et al., Macromolecules, Vol. 26, pp. 3411-3418 (1993)), uniaxially oriented PL polymers (P. Dyreklev et al., Adv. Mat., Vol. 7, pp. 43-45 (1995)) or blends thereof and a ductile polymer (U.S. Pat. No. 5,204,038; T. W. Hagler et al., Polymer Comm., Vol. 32, pp. 339-342 (1991); T. W. Hagler et al. Phys. Rev., Vol. B 44, pp. 8652-8666 (1991); Ch. Weder et al., Adv. Mat., Vol. 9, pp. 1035-1039 (1997)), liquid crystal systems (N. S. Sariciftci et al., Adv. Mater., Vol. 8, p. 651 (1996); G. Lxc3xcssem et al., Adv. Mater., Vol. 7, p. 923 (1995)) or oriented PL materials grown onto orienting substrates (K. Pichler et al., Synth. Met., Vol. 55-57, p. 454 (1993); N. Tanigaki et al., Mol. Cryst. Liq. Cryst., Vol. 267, p. 335 (1995); G. Lxc3xcssem et al., Liq. Cryst., Vol. 21, p. 903 (1996); R. Gill et al., Adv. Mater. Vol. 9, pp. 331-334 (1997)). The efficiency of PL polarizers is limited by the quantum yield of the PL material which, in principle, can approach unity (B. M. Krasovitskii et al. xe2x80x9cOrganic Luminescent Materialsxe2x80x9d (1988), VCH, Weinheim). Unfortunately, the uniaxial orientation of the formanisotropic, PL substances in the PL polarizers, which have been described in the prior art, not only gives rise to an anisotropic, that is, linearly polarized, emission, buts also to an anisotropic absorption. Consequently, only one polarization direction of unpolarized incident light is optimally absorbed and used for photoexcitation, while the other polarization direction is, at least partially, wasted. As a result, the prior art PL polarizers are still limited in brightness, energy efficiency and contrast and have to be used in conjunction with light recycling systems and cutoff-filters as disclosed in the prior art.
In summary, the above improvements have failed to yield materials and, in particular, PL materials and polarizers manufactured thereof, that efficiently convert unpolarized light into linearly polarized, chromatic light. The need, thus, continues to exist for materials and devices made thereof which, in an economical and satisfactory way, allow the efficient generation of polarized, chromatic light.
One object of the present invention to overcome the problems related to the prior art PL polarizers, is to provide PL materials and PL polarizers made thereof which are characterized in a high degree of polarization in their emission and a low degree of polarization in their absorption.
Another object of the present invention is to provide methods for the preparation of PL materials and PL polarizers thereof which are characterized in a high degree of polarization in their emission and a low degree of polarization in their absorption.
Still another object of the present invention is to provide display devices that comprises at least one PL polarizer that is characterized in a high degree of polarization in its emission and that is characterized in a low degree of polarization in its absorption.
Other objects of the present invention will become apparent to those skilled in the art in the following detailed description of the invention and the appended claims.
The present invention is based on our surprising finding that the foregoing and other objects are achieved by making and using materials that display a novel phenomenon which hereinafter is explained in detail and referred to as polarizing energy transfer. As noted heretofore, only a portion of the available energy of unpolarized excitation light is absorbed (and at most only that portion is subsequently re-emitted as linearly polarized light) by the prior art PL polarizers. Most importantly, we have now found that the properties, particularly brightness and efficiency, of such PL materials and products made thereof can be dramatically improved, by incorporating auxiliary luminescent centers or sensitizers. More specifically, we have found that in materials which comprise an appropriate, usually essentially randomly oriented sensitizer, that maximally harvests optical energy by essentially isotropic absorption, a polarizing energy transfer may occur during which the absorbed energy is efficiently transferred to a, usually uniaxially oriented, emitter which, subsequently, emits highly linearly polarized light. The general concept of sensitization in PL materials is well known in the art, and is applied in various technical applications such as lasers (U.S. Pat. No. 4,081,761) or daylight-fluorescent paints (B. M. Krasovitskii et al. xe2x80x9cOrganic Luminescent Materialsxe2x80x9d (1988), VCH, Weinheim). Usually, the sensitization in these systems arises from an electronic energy transfer process between a luminescent center which absorbs incident light and a luminescent material (usually in close proximity to the luminescent center) that subsequently (at least partially) re-emits this energy. While such classic energy transfer processes have been investigated in great detail (T. Fxc3x6rster, Ann. Phys., Vol. 2 p. 55 (1948); D. L. Dexter. J. Chem. Phys., Vol. 21, pp. 836-850 (1953); A. Gilbert et al. xe2x80x9cEssentials of Molecular Photochemistryxe2x80x9d (1991), Blackwell Science, Cambridge; S. E. Webber, Chem. Rev., Vol. 90, pp. 1469-1482 (1990); N. L. Vekshin xe2x80x9cEnergy Transfer in Macromoleculesxe2x80x9d (1997), SPIE Optical Engineering Press, Washington), the materials of the present invention exhibit a polarizing energy transfer process, which was not reported or suggested before. We have now found, that these new materials enable the fabrication of highly efficient PL polarizers. Moreover we found that the latter, when used in PL display devices, lead to significant improvement in brightness and energy efficiency of these devices compared to the prior-art.
Photoluminescent and photoluminescence are hereinafter abbreviated with the designation PL.
The designation PL polarizer refers to a, for instance, shaped material that is characterized in exhibiting photoluminescence. The PL polarizer may be of many useful forms, for example, but not limited to, a fiber, rod, film, sheet, layer, tape or plate, which may be homogeneous and continuous, and may be structured or patterned, and may comprise multiple individual elements, zones or pixels, or arrays thereof.
To clarify the operation of the devices and the conditions of experiments, the following common definitions of the several axes will be used:
The polar axis of a linear polarizer or analyzer is the direction of the electrical field vector of the light that is transmitted by the polarizer films. The PL polarizer axis is the direction of the electrical field vector of the light emitted by the PL polarizer.
Herein, the degree of emission polarization of a PL polarizer (also referred to as degree of polarization in emission) is expressed as the emission dichroic ratio (also referred to as dichroic ratio in emission) of the PL polarizer. The emission dichroic ratio is defined as the ratio of the integrated emission spectra measured through a linear polarizer (analyzer) with its polar axis parallel and perpendicular to the PL polarizer axis, using unpolarized excitation light.
Herein, the degree of absorption polarization of a PL polarizer (also referred to as degree of polarization in absorption) is expressed as the absorption dichroic ratio of the PL polarizer. The absorption dichroic ratio is defined as the ratio of the absorption measured with incident light linearly polarized parallel and perpendicular to the PL polarizer axis, measured at the wavelength used for excitation of the PL polarizer.
Herein, a sensitizer is defined as a species and/or moiety and/or domain, which, at least at one wavelength that can be used for photoexcitation, gives rise to a substantial, essentially isotropic absorption by the PL polarizer in which it is comprised; it is further characterized in that it transfers the absorbed energy, at least partially, to an emitter, if the latter is also comprised in the PL polarizer.
Herein, an emitter is defined as a species and/or moiety and/or domain, which gives rise to a significantly anisotropic, that is linearly polarized, photoemission of the PL polarizer in which it is comprised.
Herein, the excitation wavelength is defined as the wavelength that is used for excitation of a PL polarizer.
Herein the terms absorption and emission relate to optical processes.